Synthetic membranes are used for a variety of applications including desalination, gas separation, filtration, and dialysis. The properties of the membranes vary depending on the morphology of the membrane, i.e. properties such as symmetry, pore shape, and pore size, and the polymeric material used to form the membrane.
Different membranes can be used for specific separation processes, including microfiltration (MF), ultrafiltration (UF), and reverse osmosis. MF and UF processes are carried out under differential pressure and are distinguished by the size of the particle or molecule that the membrane is capable of retaining or passing. MF can remove very fine colloidal particles in the micrometer and sub micrometer range. As a general rule, it can filter particles down to 0.1 μm, whereas ultrafiltration can retain particles as small as 0.01 μm and smaller. Reverse osmosis operates on an even smaller scale.
As the size of the particles to be separated decreases, the pore size of the membrane decreases and the pressure required to carry out the separation accordingly increases.
A large surface area is generally needed when a large filtrate flow is required. One known technique to make a filtration apparatus more compact is to form a membrane in the shape of a hollow porous fiber. Modules of such fibers can be made with an extremely large surface area per unit volume. Microporous synthetic membranes are particularly suitable for use in hollow fibers and are typically produced by phase inversion techniques.
Microporous phase inversion membranes are particularly well suited to the application of removal of colloidal suspensions, viruses, and bacteria. Of all types of membranes, the hollow fiber membrane contains the largest membrane area per unit volume.
UF and MF membranes are used in separating particles and colloidal matter from liquids. In a typical scenario, water containing solutes and precipitates is passed through a bank of semipermeable tubular membranes housed in a module, often at elevated pressures. The filtered water is drawn off and collected, leaving a residue of solid material in the membrane pores or on the unfiltered side of the membrane.
It is preferred that the pores of the membrane be kept relatively free of contaminants. As the amount of pore blockage increases, the filtration efficiency of the module decreases and the amount of pressure required to maintain a viable throughput of liquid increases. As pressure increases, the likelihood of membrane rupture becomes more significant.
Under some circumstances, it may be desirable to treat water containing impurities with a flocculating agent prior to filtration. The purpose of flocculating agents is to cause dispersed colloids to coagulate and form ‘flocs’. Flocs have the advantage of entrapping smaller colloidal particles, thereby making filtration more efficient. They may also aid in the removal of dissolved particles. Under the influence of a flocculating agent, dissolved and suspended particles coagulate and precipitate from the water, thereby removing color, and turbidity.
Thus, in practice, the filtrate containing the flocculating agents, colloids, bacteria and other particulate matter is passed through the filtration unit under pressure, expelling filtered water and leaving the floe trapped within the unit, and more particularly on the waste side of the membrane and in the pores of the membrane. Flocs are particularly problematical in causing membrane blockage, and membrane performance gradually diminishes with use until it becomes necessary to clean the membranes.
One of the most commonly employed flocculating agents in the water purification field is ferric chloride, and the resultant floe is known as Fe floc. Build-up of Fe floe leads to iron fouling and eventually results in membrane performance degradation that diminishes the lifetime of these high cost membrane units. Two of the most widely used membrane compositions, polypropylene (PP) and polyvinylidene fluoride (PVDF), foul irreversibly with Fe floc and can become useless.
Residual material accumulating in and on the membrane is often removed by ‘backwashing’, that is, running the current of water counter to its normal direction of flow to dislodge the contaminants from the membrane. Gas backwashing of the membrane is also possible.
Backwashing generally involves increasing the pressure on both sides of the hollow fibers within a module a relatively high value before suddenly releasing that pressure on the unfiltered side of the membrane walls to cause a sudden pressure differential across the walls, which causes a backwash action. However, it is difficult to achieve complete removal of particulate matter, especially when flocculants have been used.
In addition to backwashing, the membranes may be de-fouled by more conventional cleaning regimes such as solution treatment with one or more of (and usually in a sequential manner) citric acid, oxidizing agents, in particular chlorine, and chelating agents such as EDTA.
Citric acid is usually regarded as a satisfactory cleaning agent, however, even it does not provide ideal levels of cleaning, and the membrane performance diminishes even following regular use/cleaning cycles. Moreover, the cleaning process usually involves a number of steps, and one or more of the steps may need to be conducted for long periods of time. Temperature control is also usually required.
Inorganic acids and bases are the mainstay of conventional cleaning agents. As well as suffering from the drawbacks mentioned above, these agents present their own problems because they may chemically attack the membranes and/or module components. Combinations of an aqueous inorganic acid, generally nitric acid, and a reducing agent, e.g., ascorbic acid, have also been used. However, none of the above regimes sufficiently de-foul membranes, particularly PVDF membranes, of the floc. Hence, there exists the need to improve the cleaning regime while at the same time avoiding the use of potentially severe cleaning agents.